Heir to the Empire was the book that, in the early 1990s, started what would become known as the Star Wars Expanded Universe, and what is now known as the Legends Universe. For those who do not know the history, in the late 1980s, Lucasfilm was trying to decide how to keep the Star Wars brand alive and thriving given that the original trilogy ended in 1983 after Return of the Jedi and George Lucas had not totally committed to doing the prequel trilogy. The end result was a decision to keep the stories and characters alive through novels, of which this one, written by Timothy Zahn, is the first.
The book is set five years after Return of the Jedi with The New Republic in somewhat of a stalemate with the remnants of The Empire. The Empire has regrouped under the command of Grand Admiral Thrawn, a mysterious Alien commander of the Empire's new flagship who has second-to-none strategic and analytical capabilities and studies the artwork of his adversaries to predict their actions. Leia and Han are married, Leia is pregnant with twins (which is, of course, a major change from the new canon material), and Luke is still trying to piece together the history of the Jedi in hopes to restore the Jedi order. Thrawn launches a plan to take down The New Republic, which involves attempts to capture Luke and Leia and sabotage the rest of the New Republic Leadership. Along with Thrawn, this book introduces the character of Mara Jade, who would become a critical character in subsequent books, and Talon Karrde, a smuggler with a very "New Hope" Han Solo vibe.
This book was originally published in 1991, and this version, with annotations from the author and editor, was published in 2011 and then reprinted in hardcover after the Disney takeover of Lucasfilm. There is a pretty extensive forward in which the history of how the books came to be is laid out, and the directives from Lucas about what stories could and could not be told. For example, none of the legacy characters could be killed and none of the books could touch on Anakin's pre-Vader life. The annotations give a lot of good insight into how the story was put together and evolved over the writing of the book. Some elements from the book, like the cloning aspects have been incorporated into the new canon material, and as many probably know by now, Thrawn was brought into the canon by a series of new books by the same author and an appearance in the animated series Rebels. At the end of the book, there is a short story titled "Crisis of Faith" which is a Thrawn-centric story that does not involve any of the legacy characters.
This, and the subsequent novels, Dark Force Rising, and The Last Command, were the stories that many of us hoped would be a sequel trilogy. But, the reality is, if Lucas ever was going to make more movies, it was going to be the prequels telling Anakin's story (which the forward of this book makes clear), and by the time those movies were made, the original cast members would have aged out of being able to make this story into a movie. Plus, the chances of Harrison Ford returning for a new trilogy of movies when he wanted Han killed off in Empire and Jedi were slim to none. It would be nice if the other two novels in the trilogy would get the same special editions released. Unfortunately, however, Zahn has said that at this point, Del Rey books has not shown any interest in releasing special editions of those books. If you are a fan of Star Wars, this is a must-read as it is one of the best stories told in the Star Wars Universe, Canon, or Legends.